This invention relates to a vibration damping composite laminate of the constrained layer damping type comprising two layers of metal and a layer of a viscoelastic polymer composition closely interposed between the metal layers, wherein the polymer composition has electrical conductivity so that the laminate can be spot-welded to metal structures.
Nowadays noises emitted from industrial machinery and structures and road vehicles have become a serious social problem, and therefore there is a keen demand for noise suppressing or reducing measures in various aspects. So far the prevailing practice of noise suppression or reduction is to use several kinds of functionally different materials such as sound insulating materials, sound absorbing materials, vibration-proof materials and vibration damping materials in selected combinations.
Meanwhile, in the current automobile industry another matter of great concern is reduction in the car weight to cope with severer standards of fuel mileage and the energy supply problem in the near future. Accordingly there is an increasing trend toward the reduction in the thicknesses of steel sheets for various panels in the car body and also toward the substitution of steel materials by lightweight materials such as aluminum alloys and plastics. Since such measures for reduction in weight lead to increasing vibrations of automobiles and, hence, to the emission of greater noise, it has become a task of great importance to develop effective and practical means to reduce vibrations automobiles.
The outcome of recent research and development in the vibration suppressing techniques includes some vibration-proof alloys and vibration damping composite structures produced by pasting a vibration damping polymeric material to a metal base or by sandwiching a viscoelastic polymer between two layers of metal. In general, a measure of the damping efficiency of a vibration damping material is chosen from loss factor (.eta.), logarithmic decrement (.DELTA.) and sharpness of resonance (Q), which are interrelated physical characteristic values. Among these characteristic values, loss factor (.eta.) is most frequently employed. Until now it has been accepted that the effect of damping vibrations is appreciable if loss factor takes a value greater than about 0.05, but the development of vibration damping materials that exhibit greater loss factor values is wanted as the noise regulations have been tightened.
In vibration damping composite materials of the constrained layer damping type produced by closely interposing a polymer layer between two metal layers, the loss factor is significantly dependent on temperature and peaks at a certain temperature. Therefore, it is necessary to selectively use a polymer which is suited to the temperature range in which the vibration damping material is to be used. Our past studies have revealed the possibility of adjusting the temperature at which the loss factor becomes maximum by using a suitable plasticizer or differently working additive. Nevertheless, it is a matter of course that a truly good vibration damping material is one which is high loss factor and is inherently low in the dependence of its damping effectiveness on temperature. Also it is accepted that a vibration damping material high in the value of loss tangent (tan .eta.) is advantageous.
Another type of vibration damping composite laminates is the extensional damping type. A laminates of this type consists of a metal base layer and a viscoelastic polymer layer which adheres to the metal layer and exists as a "free-layer" with no constraining layer thereon. In general a polymer with high complex modulus (E") is recommended for this use. However, recently vibration damping laminates of the constrained layer damping type are attracting greater interest because of comprising a relatively small quantity of polymer and being superior in vibration damping effectiveness.
From a practical point of view, vibration damping composite laminates are desired to be comparable to a steel sheet in physical properties other than vibration damping capability, and particularly in workability such as formability by bending or pressing and weldability. For applications to automobiles, spot-weldability is a particularly important factor. However, when the polymer layer in a vibration damping composite laminate lacks electrical conductivity as is usual, spot welding of such a laminate to a metal structure is difficult. If spot welding is wished, there is the need of taking a certain measure such as providing an extra short-circuiting or by-passing circuit or locally deforming the laminate by a punch so as to bring the two metal layers of the laminate into contact with each other in advance of the welding operation.
To obviate such troublesome measures, it has been proposed to render the polymer layer in a vibration damping composite laminate electrically conductive by the addition of a conductive material to the polymer, e.g. in Japanese patent applications provisional publication Nos. 50-79920, 53-128687 and 57-146649. However, the composite laminates according to these proposals are relatively low in loss factor.
As a viscoelastic polymer composition effective for damping vibrations of metal structures, Japanese patent application publication No. 54-23489 proposes a blend of a polyvinyl acetal resin and a plasticizer. However, the inclusion of a relatively large amount of plasticizer results in lowering of the strength of adhesion of the polymer layer to the metal layers of the composite laminate.